Motor standards

There has been a gradual process of change from countries using their own Standards to the adoption of European and In­ternational Standards to ensure uniformity in the widest interna­tional meaning. This process is continuing, in particular with the advent of the European Union and associated legislation.

There are already established standards that are recognized throughout Europe and beyond. The basis of most Standards originates with the International Electro technical Commission (IEC), which are then adopted either as National Standards or as European Standards. National committees throughout Eu­rope play a large part in drafting and agreeing the contents of the standards either through the IEC or the European Commit­tee for Electro technical Standardization (CENELEC). Coun­tries worldwide recognize the work of the IEC and IEC Publications often form the basis of national standards.

Because of the involvement within Europe of IEC, CENELEC, CEN and national standard bodies, for example the British Standards Institution (BSI) in the United Kingdom, there tend to be standards published with three types of identification sys­tems (International Standard — IEC, European Standard — CENELEC and National Standard for what are often the same basic standard.

The IEC Publication IEC 60034 is a good example of the variety of designations that can arise from the publication of the many parts that make up this Standard. The main motor Standard within Europe is IEC and after national agreement parts of this standard have become European Standards under CENELEC. Some parts before agreed by CENELEC were used as the ba­sis for national standards. In addition parts of IEC 60034 ap­peared as Harmonization Documents (HD) under CENELEC control.

The British equivalent of I EC 60034 is British Standard BS4999 and this itself had many parts when first issued. When re-is­sued from 1987 onwards, some parts were combined and the part numbers were adjusted to line up with IEC 60034 part num­bers where appropriate. But to avoid confusion with the original part numbers the new part numbers commenced at Part 101 with 100 added to the IEC part number where it applied.

Standards are used wherever possible for the principle motor dimensions to ensure interchangeability. This applies particu­larly to the main fixing dimensions and the shaft end. Standard dimensions are covered by IEC Publications IEC 60072-1 (small and medium size motors)and IEC 60072-2 (medium and large size motors). These also give standard symbols for each significant dimension.

British Standard BS 4999:Parts 103 and 141 are related to these IEC publications and have some additional symbols and standard dimensions which are included in the figures below where appropriate. Dimensions are generally based on pre­ferred numbers but there are some dimensions that are a carry-over from imperial measurements. The Standards in­clude tolerances for all dimensions that affect interchange­ability.

For frame sizes from 56 up to 400 inclusive, standard dimen­sions uniquely define the motor, but for larger motors this is im­practical because of a number of design constraints. Standard dimensions are primarily intended for low voltage induction mo­tors. For motors of 355 size and above there is a set of pre­ferred dimensions — the overlap of the 355 and 400 sizes with standard dimensions allows for special designs and motors other than induction motors.

There is international agreement on the nomenclature of small motors from 56 to 400 sizes inclusive. This is extended to cover larger motors in a modified form with the 355 and 400 sizes in­cluded when these are not to standard dimensions.

It is still possible to obtain some small motors to imperial dimen­sions, as specified in British Standard BS 2048:Part 1. The frame size is based on the shaft centre height multiplied by 16. For example, a motor with a shaft centre height of 3 in is a 48 frame size. Frame sizes 36,42,48,56 and 66 are available and should be prefixed with the letter B — this should avoid confusing the imperial and metric 56 sizes.

For motors below the metric 56 frame size there are no univer­sal standard dimensions. This covers the majority of small DC and AC motors. Consequently manufacturers of these motors have their own frame size conventions and dimensions to suit their products. However, most base the frame size on the frame diameter, and where motors are fitted with a square flange, this is often the flange main dimension.

Small motors, particularly of the induction motor type, are inter­nationally recognized by the frame nomenclature which gives the basic enclosure type, the size and method of mounting. This does not replace the IP, IC and IM codes which give a more detailed description of the motor, but serves to readily identify the common types by means of a simple nomenclature.

The system described in IEC Publication IEC 60072-1 consists of number/letter combinations to denote the centre height for motors with feet, the shaft diameter and/or the flange size. A motor with normal feet is designated by the centre height of the shaft above the base of the feet in millimetres followed by a let­ter denoting the frame length as either “S” for short, “M" for me­dium or “L” for long followed by the shaft diameter in millimetres, for example 112 M 28.

Flange-mounted motors can be of three basic types denoted by the letters FF for flange with clearance holes on a pitch circle di­ameter greater than the spigot diameter, FT for flange with tapped holes but otherwise as FF flanges and FI for flanges with tapped holes but the pitch circle of the holes inside the spigot di­ameter.

These letters follow the shaft diameter and are themselves fol­lowed by the flange fixing-holes pitch-circle diameter in milli­metres, for example 28 FF 215. In cases where a motor has both feet and a flange the designation appears as 112 M 28 FF 215, for example.

The basic system outlined in British Standard BS 4999 : Part 103 differs from the IEC Standard and consists of a letter, num­ber, letter combination of which the meanings are as follows:

A) First letter to indicate the basic enclosure either as “C” for enclosed ventilated or “D” for totally enclosed. (It should be noted that the letter “E” has been used to indicate flameproof enclosures but this is not covered by the stan­dards. When the system is extended to large motors an extra letter is often added to indicate a particular variant, for example “DW” for totally enclosed, water cooled or as a range identifier, for example “GD” for the manufacturer’s G range of totally enclosed motors.)

B) Number of two or more digits indicating the centre height of the shaft above the base of the feet of horizontal motors in millimetres. For flange-mounted motors or others with­out feet, the same basic frame size retains the same num­ber. The numbers are from the R20 preferred number series except for the 132, which is approximately halfway between 125 and 140.

C) First suffix letter to characterize the longitudinal dimension where more than one length is used, specified as either “S” for short, “M” for medium or “L” for long. (Some large motors using the same basic system have had additional letters added by some manufacturers to indicate a further length step, for example “MX” as a length between “M” and “L”.).

D) For other than foot-mounted motors an additional letter to indicate the type of mounting as either “D" for flange, “V” for skirt, “C” for face flange, “P” for pad or “R” for rod. (The “P” mounting can usually be used for rod mounting.)

As an example a motor of the 180 size, of an enclosed venti­lated type, with a medium length and for flange mounting would be called a C180MD.

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